Because, photoautotrophs serves as their carbon sources. Chemoheterotrophic bacteria are those bacteria that are incapable of producing their own food, they depend on photoautotrophs, which are capable of making their own food by trapping energy from the sun. Thus, photoautotrophs serves as source of food for the chemoheterotrophic bacteria.
The usual problem that would frustrate attempting to organize entire world for the study is we do not know how many separate species of life forms that usually would exist on this planet. Hope this is the correct answer and this would help.
